plastic injection mold jetting, meld lines

Jetting

Jetting is a result of the inertial effects of material exiting at a very high velocity from a restrictive gate.

Jetting is caused by restricted gates and poor placement of gates in the mold. The material must have something to stick to, or impinge on, as it enters the cavity. If a restricted gate is in-line with a relatively thick cavity (as shown below) the inertial effects of the fast flowing plastic will cause it to “jet” into the cavity. This might also happen because of the injection rate being too high for the viscosity of the material. Jetting could be remedied by adjusting the design of the gate or moving the gate to a better location.

Meld Lines

Meld lines are formed when two separate flow fronts meets. These are preferred over weld lines as they will normally result in improved cosmetics and structure. In a meld line, the plastic comes together at an angle greater than 135 degrees. As compared to a weld line, the flow fronts come together at less than 135 degrees. If the cavity has unbalanced flow paths unnecessary meld lines can occur.

The figures below shows a part with a hole/window in it. The middle figure shows a gate location which results in a weld line. The figure on the right shows the formation of a meld line.